This invention relates to a laminate for a safety glazing and more particularly to a laminate which is shrink-stable during formation of such a glazing.
It is known to use an energy absorbing interlayer of plasticized polyvinyl butyral (PVB) with one or more rigid layers such as glass in a safety glazing. Such a glazing is usually prepared by interposing the PVB layer between glass sheets while eliminating air from between the engaging surfaces and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB and glass and form an optically clear structure. These glazings are used in windows such as the front, side and rear windows in motor vehicles, particularly windshields, where the interlayer can absorb a blow from the head of an occupant without penetration of the windshield.
In recent years additional sophisticated features are appearing in such windows to enhance performance. These include special, layered metal/dielectric stacks for solar radiation control which may be electrically conductive for defrosting, defogging, etc; holographic layers as solar reflecting mirrors and in head-up displays to facilitate viewing instruments on the vehicle dashboard while looking straight ahead; photochromic and electrochromic layers which controllably change color on exposure to solar radiation or application of a voltage; layered protective antilacerative structures on the inboard side of a conventional three layer glass/PVB sheet/glass laminate to minimize lacerations from sharp edges of broken glass; special plastic layers in bilayer structures replacing one glass layer of such a three layer glass laminate, and similar, functional performance-enhancing layers and coatings. These performance layers are usually deposited on or adhered to a carrier layer which is different from the low modulus, elastomeric PVB which is unsuitable as a carrier. For use in safety glazings a carrier layer should have good clarity, be relatively uniform in thickness and strong having high modulus to facilitate ease of handling and processing during association with the performance layer(s). Frequently biaxially oriented polyethylene terephthalate is used as noted, for example, in U.S. Pat. No. 4,465,736.
Concurrent with these performance advances, window shapes are appearing having severely curved and angled configurations serving, for example, to minimize aerodynamic drag and enhance ability to see from within the vehicle. When forming a safety glazing of such complex curvature having a PVB layer and a carrier layer of the type referred to, problems occur with the carrier layer. More specifically, when planar plastic layers on or between rigid sheet(s) such as glass having the desired complex curvature are heated to bond the PVB to the rigid sheet(s), the flat carrier layer cannot perfectly conform to the complex curvature without stretching, in the absence of which it forms wrinkles, folds or pleats (collectively "wrinkles"), usually, though not necessarily, in one or more sections near the periphery of the glazing laminate. These visually apparent wrinkles are an optical defect in the safety glazing. Wrinkling is not encountered with conventional safety glazings using only plasticized PVB which readily flows between the rigid sheets and evens out in thickness during autoclave laminating.
To avoid wrinkling, published European Application No. 0304898 discloses forcing a planar collection of plastic layers secured to each other against and adhesively pres bonding them to a curved glass layer followed by autoclave bonding of the assembly to form the safety glazing. This approach combines plastic film forming with glass bonding and handling which are quite different operations usually found separated in the safety glazing industry where film manufacture and supply is by a plastic fabricator and glazing manufacture by a glass laminator.
It would be desirable to provide a layered plastic prelaminate which could be interchangeably used in the same way as a single layer of plasticized PVB in conventional autoclave laminating to provide a safety glazing with enhanced properties where unsightly wrinkling is avoided.